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Jeep Xj Running Rich

Discussion in 'Quitters' Club' started by DanStew, Aug 9, 2021.

  1. Aug 9, 2021
    DanStew

    DanStew Incorrigible. Staff Member

    Lexington, South...
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    My son got himself an 1999 XJ this weekend. It was a friend of mines and she was moving and she gave it to him. I have worked on this XJ a few times. It has been sitting in the backyard for a year. i put in a new battery and added some fresh fuel and it started up and ran fine. We drove it 15 miles to home. i was watching the XJ and after a while i noticed it was running rich, black smoke coming out upon acceleration. My son did not notice any drive ability problems. I kind of expected the injectors would be gummed up. Well today we pulled the plugs and they were all black and sooty. So i am disregarding the injectors now since all plugs looked identical. The check engine light is not on. The air filter is a KN and is kind of cruddy, and that will need cleaning. The MAP sensor is suspect and the upstream O2 sensors seems to be an issue for people but i am still not throwing a code. It is the 4.0l

    Am I missing anything else to look for? I will eventually do a compression test, but the rig does run real good.
     
    Last edited: Aug 9, 2021
  2. Aug 9, 2021
    SFaulken

    SFaulken Member 2021 Sponsor 2020 Sponsor

    Marysville, WA
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    Engine Coolant Temperature and Air Intake Temperature senders. I can't recall on the XJ's, but I think there's one ECT that talks to the computer, and one that runs the gauge. If it's not reading correctly, and the computer never thinks the engine is getting warmed up, it never comes off the cold start enrichment, and stays in open loop operation, which means running rich as hell. I *think* the Air Intake Temp sender can cause similar issues, but don't quote me on that.
     
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  3. Aug 10, 2021
    Jeepenstein

    Jeepenstein Me like Jeep..

    North Central FL
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    Jun 15, 2003
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    I'd start with the upstream Oxygen sensor.. Also take a look at fuel rail pressure to make sure the regulator is doing it's thing..
     
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  4. Aug 10, 2021
    Downs

    Downs Kubota Orange Rattlecan

    Hunt County Texas
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    Jul 25, 2020
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    Throw that K&N in the garbage and put a standard paper element filter in there. A lot of times these sensors can be out of range and still not throw a code.

    I would be suspect of the battery cables and grounds. If you don't know if they've ever been replaced and cleaned I'd start there as all these sensors use a 5V reference signal and dirty grounds and internally corroded battery cables can cause all sorts of issues that will leave you chasing your tail forever.

    Get the FSM (I may have a copy of a 97 and a 00 XJ FSM) and it will have test procedures for the various sensors, the sensors mentioned so far can all be tested with a multimeter.

    On a 99 he'll only have one sensor and it's in the thermo housing. 96 and older had two, one in the the thermo housing for computer/fan use and another in the back driver's side of the head for feeding data to the cluster.

    AIT sensor can cause issues as well.

    All of these sensors have testing procedures involving a multimeter and a resistance readings based on temps. They can be a bit out of range and not trip a CEL.
     
  5. Aug 14, 2021
    DanStew

    DanStew Incorrigible. Staff Member

    Lexington, South...
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    I checked the MAP sensor and it is functioning correctly. i installed the original filter housing, just waiting for my son to get a filter for it, then we will drive around and see if the filter may have fixed the issue.
     
  6. Aug 15, 2021
    Downs

    Downs Kubota Orange Rattlecan

    Hunt County Texas
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  7. Aug 15, 2021
    PeteL

    PeteL If it wasn't for physics, and law enforcement... 2021 Sponsor 2020 Sponsor

    Hills of NH
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    O2 sensor in the exhaust. (?)
     
  8. Aug 16, 2021
    Lockman

    Lockman My " Rosie" 2021 Sponsor 2020 Sponsor

    White City, NY 14617
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    Vacuum leaks ? Check all the Hose connections.
     
  9. Aug 28, 2021
    DanStew

    DanStew Incorrigible. Staff Member

    Lexington, South...
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    well happy to report the engine does not seem to be running rich now. i did repalace the air filter with factory housing and filter.
    My son has been driving it a bit now and i pulled a plug today to see if it was sooty and it was really clean.
     
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